A variety of incandescent lamps exist for specialized purposes. Once such specialized purpose is a traffic signal lamp in which the lamp is mounted on a fixture which is generally located above the line of sight. Consequently, the filament of such a lamp is designed so that when it is placed in its fixture it will radiate light downward rather than upwardly, where it would be wasted. One such lamp uses a W-shaped filament with the bottom portion of the W located below the central medial plane of a spherical shaped envelope. For traffic purposes, the lamp can either be of clear glass with a filter, such as a colored glass filter, placed in front of it so that the appropriate color would be transmitted, i.e. red, green or yellow. In other types of lamps, the lamp itself is colored, generally with a painted organic pigment color over the lamp envelope.
Work has also been done in connection with improving the efficiency of incandescent lamps by applying to the lamp envelope a visible transmissive-infrared reflective (heat mirror) coating. The envelope of such a lamp is often optically shaped and the coating placed therein will reflect back to the filament a substantial portion of the IR energy that is produced to raise its operating temperature. This in turn reduces the amount of power needed to heat the filament to its operating temperature, thereby increasing the efficiency of the lamp.
The heat mirror coating also transmits a large portion of the visible range energy produced by the filament. One such type of lamp is shown, for example, in the Thorington, et al. U.S. Pat. No. 4,160,909, which is also assigned to the assignee of the subject application in which the coating is a composite of three discrete films of Ti0.sub.2 /Ag/Ti0.sub.2 which is capable of transmitting an average over the the visible range of about 60% and above of the visible range energy and reflect an average of about 60% and above of the IR range energy. Other types of such lamps also have been proposed using various other types of coatings than that disclosed in the Thorington, et al patent. Another incandescent lamp using a different type of coating is disclosed in application Ser. No. 45,645, filed June 5, 1979 in the name of Peter Walsh, which is a continuation of application Ser. No. 863,155, filed Dec. 22, 1977, now abandoned, both of which are also assigned to the assignee of the subject application. In that application, the coating is an etalon of a dielectric film sandwiched between two films of silver.
In lamps of the type using a heat mirror coating, theoretically a point source filament precisely located at the optical center of a spherical envelope, for example, would be ideal so that the maximum amount of IR energy reflected by the coating will impinge back onto the filament. However, a point source filament is not reliable and, instead a "compact" filament is used. The term "compact" is meant to mean an elongated filament in which the length to diameter ratio of the filament is made relatively small. Such filament is generally mounted vertically in the envelope with respect to the lamp base.
The use of such a lamp with a heat mirror coating and "compact" filament in a specialized environment, such as a traffic signal lamp, would be somewhat inefficient. Although the overall efficiency of the lamp has been raised by the coating, the light emitted by the filament would not be preferentially directed downwardly. Also, from the point of view of operating life, in general service types of lamps as well as in traffic signal lamps, a compact filament is not as desirable as a C-shaped (or circular shaped) filament, which is the type of filament usually used in general service lamps. Such C-shaped filaments have three mounting supports, one at each end and the third in the center and is quite rugged.
If a C-shaped or circular filament were used in a spherical-shaped envelope having an IR reflective coating, the lamp would be inefficient since all of the filament would be located far from the optical center of the envelope and the IR energy would not be optimally reflected back to the filament.
Accordingly, the present invention is directed to a novel incandescent lamp having a visible transmissive-IR reflective (heat mirror) coating on the envelope in which a filament, such as a C-shaped or circular filament, is used. The envelope is uniquely shaped as an ellipse which is rotated about its center with the two foci of the ellipse forming an infinite number of foci lying in a circle called the focal circle. The filament is located on or near the focal circle so that the energy reflected by the heat mirror coating impinges on it.
The lamp can be utilized with either a heat mirror coating which can transmit light over the entire visible range, or it can be used with a coating such as to produce a selective color. The latter improves doubly the efficiency of the lamp both from the point of view of the IR reflective coating increasing the energy efficiency and the selective color coating being more efficient than a pigment coating and thereby reducing the amount of energy needed to produce a given amount of light at the particular color.
It is, therefore, an object of the present invention to provide an incandescent lamp utilizing an envelope in the shape of an ellipse of revolution with the filament being located on or near the focal circle of the envelope.
A further object is to provide an incandescent lamp having a curved filament located on or near a focal circle of an envelope shaped as an ellipse of revolution, with the lamp having a coating thereon to reflect IR energy back to the filament.
An additional object is to provide an incandescent lamp having an envelope in the shape of an ellipse of revolution with a curved filament located on or near a focal circle defined by the ellipse, with the coating also transmitting only a selected color portion of the visible light.